1. Field of the Invention
The invention is directed to formulations for treating hypersensitive teeth and to methods for making and using such formulations.
2. Description of Related Art
Hypersensitive teeth can cause pain and discomfort when subjected to changes in temperature, pressure or chemical action. Exposure of the dentin frequently leads to hypersensitivity. Dentin exposure may occur due to recession of the gums, periodontal disease or improper dental care.
A significant breakthrough in the treatment of hypersensitivity came with U.S. Pat. No. 3,122,483 to Rosenthal. This patent reported that certain strontium compounds, especially strontium chloride, acted as desensitizing agents. Later, U.S. Pat. No. 3,863,006 to Hodosh reported that potassium nitrate could act as a desensitizer, and U.S. Pat. No. 4,631,186 and U.S. Pat. No. 4,751,072, both to Kim, reported that potassium chloride and potassium bicarbonate were also satisfactory desensitizers.
Ionic salt desensitizers, such as potassium nitrate and strontium chloride, appear to act by penetrating hypersensitive teeth through "tubules" in the dentin of the tooth. Dentinal tubules are small open tubes in the dentin that facilitate energy and material transfer between the interior and exterior of the tooth. The ionic desensitizers apparently interfere with the operation of the nerve in the tooth, reducing the nerve activity, and thereby reducing the sensitivity of the tooth.
Examples of ionic salts as desensitizers also include U.S. Pat. No. 3,699,221 to Schole et al., issued Oct. 17, 1972. This patent is directed to an early system for desensitizing teeth. The patent is directed to a strontium chelate in dentifrice, tooth powder, or chewing gum form. (A number of ingredients are listed in the toothpaste formulation, including 1% by weight "algin" in two examples. The purpose of the algin is not made clear.)
While the desensitizing properties of ionic desensitizers are excellent, such desensitizers are not "permanent," in that they do not remain in proximity to the tooth nerve forever, and the desensitizer must be reapplied to the tooth continually. In an effort to sustain the administration of an ionic desensitizing salt to the tooth, U.S. Pat. No. 5,188,818 to Merianos et al., issued Feb. 23, 1993, described a strontium salt of a maleic anhydride-methyl vinyl ether copolymer as a desensitizer. The polymer apparently adheres to teeth and releases the strontium over time.
A second class of desensitizing materials has recently been developed. This class of desensitizers acts to occlude or otherwise seal off some or all of the tubules present in the dentin and so reduce material and energy exchange between the interior and exterior of the tooth. These materials may have more or less affinity for the material in the dentin tubule and may exhibit more or less permanence as an occluding agent. Some materials migrate to the tubule quickly and are very prompt in occluding tubules, while other materials take time to develop as occluding agents. The materials may also remain in the tubules for varying amounts of time. Thus, the degree of occlusion of the many tubules found in dentin and the rapidity of the onset and the rate of decline of meaningful occlusion are important aspects of such occluding agents.
An example of an occluding agent is found in U.S. Pat. No. 5,270,031 to Lim et al., issued Dec. 14, 1993. This patent is directed to a tubule occluding desensitizer comprising a polyacrylic acid such as Carbopol.RTM. polymeric materials (B.F. Goodrich). Another tubule occluding composition is disclosed in U.S. Pat. No. 5,374,417 to Norfleet et al., issued Dec. 20, 1994. That patent discloses a potassium salt of a synthetic anionic polymer, such as a polycarboxylate.
One important aspect of desensitizing agents, whether ionic salts or tubule occluding agents, is that the agent must not be toxic or otherwise harmful to persons being treated for hypersensitivity. Preferably, a good desensitizing agent should be compatible with other oral care ingredients, such as conventional dentifrice or mouthwash ingredients, so that the agent may be administered in a conjunction with other dental therapies and prophylaxis. The organoleptic qualities of the desensitizing agent should also be pleasant to encourage compliance.
Although alginates have been known for some time as binders in dentifrice applications, we have now discovered that alginate salts are excellent occluding agents and meet all the requirements for a good desensitizing agent.
Alginates are salts of alginic acid, a polysaccharide having a molecular weight of about 240,000 daltons. The polysaccharide is extracted from giant brown seaweed and kelp. Alginic acid has been described as a linear polymer of .beta.-(1-4)-D-mannosyluronic acid and .alpha.-(1-4)-L-gulosyluronic acid residues. The exact proportion of the monomers depends on the source of the polymer. The salts, typically sodium salts, of the acid are used as gelling agents and as thickeners in dentifrice applications and ice cream.
The use of alginate in a dentifrice system is demonstrated in U.S. Pat. No. 4,401,648 to Piechota. Jr., issued Aug. 30, 1983. This patent is directed to a dental creme containing a gel system of xanthan and alginate. The ratio of xanthan and alginate in the system is 1:3 to 3:1 and the system comprises 0.2-5% of the dental creme.
Alginates have also been used in conjunction with anti-caries agents as in U.S. Pat. No. 4,775,525 to Pera, issued Oct. 4, 1988. That patent is directed to a composition for reducing dental plaque. The essential ingredient of the patent is a calcium ion chelating agent comprising sodium alginate. Although the patent is directed to caries control, the composition may also contain a desensitizing agent such as strontium chloride.
U.S. Pat. No. 4,855,128 to Lynch et al., issued Aug. 8, 1989, is directed to plaque inhibitory agents in mouthwashes, toothpastes and other oral care systems. These agents are present from about 0.0025% to about 2.000% on a weight to volume basis, and are polysaccharides selected from xanthan gum, gum tragacanth (pectin), guar gum, gum karaya, chondroitin sulfate, polygalacturinic acid, sodium alginate and carrageenans of the kappa/lambda configuration. These ingredients apparently inhibit the coaggregation of bacteria and so inhibit the formation of plaque.
Alginates have been used in dentifice formulations for other purposes as well. The permeability of the enamel surface of teeth was reported in Tung, M. S. and Brown, W. E., "Characterization and Modification of Electrochemical Properties of Teeth," J. Dent. Res. 62(1):60-64 (Jan. 1983). The article reported that the permeability of teeth could be modified by anionic compounds such as phytate and alginate and by cationic compounds such as protamine, polyarginine, and polylysine, or by an alternating coating of these compounds. The purpose of this study was to investigate the behavior of incipient caries, which was reported to form below the enamel surface.
Another occluding agent is found in U.S. Pat. No. 4,645,662 to Nakashima et al., issued Feb. 24, 1987. This patent is directed to an oral composition for treating dentinal hypersensitivity. The composition contains aluminum and a carboxylate compound which combine to form a composition that occludes the dentin tubules. Among the binders that may be used in a dentifrice containing the occluding compounds are: carrageenan, cellulose derivatives (e.g., sodium carboxymethylcellulose, methylcellulose, hydroxyalkylcelluloses and sodium carboxymethyl-hydroxyethyl cellulose), alkali metal alginates (e.g., sodium alginate), alginic acid-propylene glycol ester, gums (e.g., xanthan gum, tragacanth gum, karaya gum, and gum arabic), synthetic binders (e.g. polyvinyl alcohol, sodium polyacrylate, carboxy vinyl polymer, and polyvinyl pyrrolidone), and inorganic binders (e.g., gelling silica, gelling aluminum silica, magnesium aluminum silicate (sold under the trade name Veegum), and synthetic hectorite clays such as Laponite (trade name)). The binder is added in an amount of 0.1 to 10% by weight, and preferably 0.2-5% by weight. The only reported example using an alginate, however, used one percent alginate and obtained unacceptable results. (See Example 2 and formulation #4 in Table 5.)
Despite the presence of occluding agents and other desensitizing agents, the search continues for an occluding agent that has the proper mix of desensitizing and organoleptic qualities to provide a meaningful therapy for treatment and relief of hypersensitivity while maintaining a pleasing "mouth-feel" for the therapeutic product. Alginates have not heretofore been recognized as desensitizing agents, even in desensitizing formulations.